The color of paint depends on the color you want to paint your army. One of the simpliest ways to get an effective looking force is the use of Triads. Some companies sell then already packaged for you, other you pick and choose and then go with it.
Triads are basicly 3 paint colors, one base color, one shade color, and one highlight color.
So if you were going to do blue as your base color, you would choose a color of blue you liked for your base, then select a much darker blue color for a shade and a brighter, lighter color of blue for the highlight.
The color of primer will dictate how the models looks once it is painted. Blacks makes it look darker, grey is the most natural, and white is going to make it brighter.
Colored primers are alternatives if you want to get something done fast and the color works for you. I've used the army painter primers before and they are pretty good. For the most part I use grey or white primer. Its more of a comfort thing than anything else. I'm used to using them and it works just fine for me. But like I said, I have used the colored and black primers as well. For me, the primer choice is more a choice based on what I want to do with my models.
Vallejo also has a line of "foundation" type colors as well. I have been using these and I have to say I am really impressed with them. I like them better than the original foundation colors, but their colors aren't as vibrant. Still, they do make a great shade color or place to start and then build on.
Reaper also has a line of foundation type paints. The colors are more vibrant and I like them as well.
also what helps in making your selection is the use of complementary and contrasting colors. Visually, they look good to the eye.
An Example is if you wanted to paint your models primarily green, the contrast color to green is red, so little details or a second color to use could be red.
Use of a color wheel will help selecting colors that work well together. You can find color wheels online easily and then kind of go from there.
Just because I say red goes with green, doesn't mean you have to use red, you could use a complementary color like yellow or purple. There are a lot of options.
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